Welcome to a very special holiday edition of Sciency Words! Today’s science or science-related term is:

FROST LINE

When a new star is forming, it’s typically surrounded by a swirling cloud of dust and gas called an accretion disk. Heat radiating from the baby star plus heat trapped in the disk itself vaporizes water and other volatile chemicals, which are then swept off into space by the solar wind.

But as you move farther away from the star, the temperature of the accretion disk tends to drop. Eventually, you reach a point where it’s cold enough for water to remain in its solid ice form. This is known as the frost line (or snow line, or ice line, or frost boundary).

Of course not all volatiles freeze or vaporize at the same temperature. When necessary, science writers will specify which frost line (or lines) they’re talking about. For example, a distinction might be made between the water frost line versus the nitrogen frost line versus the methane frost line, etc. But in general, if you see the term frost line by itself without any specifiers, I think you can safely assume it’s the water frost line.

Even though our Sun’s accretion disk is long gone, the frost line still loosely marks the boundary between the warmth of the inner Solar System and the coldness of the outer Solar System. The line is smack-dab in the middle of the asteroid belt, and it’s been observed that main belt asteroids tend to be rockier or icier depending on which side of the line they’re on.

It was easier for giant planets like Jupiter and Saturn to form beyond the frost line, since they had so much more solid matter to work with. And icy objects like Europa, Titan, and Pluto—places so cold that water is basically a kind of rock—only exist as they do because they formed beyond the frost line. This has led to the old saying:

Okay, maybe that’s not an old saying, but I really wanted this to be a holiday-themed post.

When Voyager 1 trained its camera on the moons of Jupiter, scientists back on Earth had no idea what to expect. Turned out they were right. Voyager was snapping photos of geological features unlike anything anyone had ever seen before. Which meant it was time to make up some new sciency words!

FACULA

Last week, we learned about the word macula (plural, maculae): a special term for dark spots on the surface of a moon or other planetary body. Now if you’re going to invent a special term for dark spots, you really ought to have a term for bright spots too. And that term is facula (plural, faculae).

To an ancient Roman, facula meant “little torch.” To a modern planetary scientist, it refers to a surface feature that looks brighter than the surrounding terrain. The term was first used this way to describe bright, circular features seen on Ganymede.

If you think Ganymede’s faculae look a little like craters, you’d be on the right track. Like most moons in the outer Solar System, Ganymede is composed of a mixture of rock and ice, and it may have a layer of liquid water beneath its surface.

So the craters left by asteroid impacts on Ganymede sometimes get filled in with icy slush. The slush freezes, and the crater is virtually erased. Only the crater rim remains, and you can see a color difference between old and new surface ice.

The term facula can be used to describe almost any bright spot on a planet-like surface, not just resurfaced craters. For example, there are faculae on the dwarf planet Ceres. Ceres’s faculae are still being investigated by the Dawn spacecraft, but the current best guess is that they’re salt deposits—perhaps salt left behind after very briny water boiled into space.

For next week’s edition of Sciency Words, we’ll conclude our visit to the moons of Jupiter with a quick trip to Io.

Bonus Sciency Word: An impact crater that gets filled in and smoothed over, like the craters on Ganymede, is also called a palimpsest.